Cellular telephony is based on the concept of dividing a geographical area into a plurality of individual sub-areas or "cells." Situated within each cell is a base station containing at least one receiver and transmitter tuned to a particular frequency (channel) for communicating with a mobile transceiver (i.e., terminal), also tuned to receive and transmit on that channel. In actuality, the receive and transmit portions of the channel are spaced 45 MHz apart. Typically, the mobile terminal is carried in a vehicle in transit within that cell. As the vehicle containing the mobile terminal travels from one cell into another, the call between the mobile terminal and the cell site base station is transferred to a base station of the new cell. Typically, the new cell site base station has its receiver and transmitter tuned to a different channel, necessitating that the mobile terminal change its reception and transmission channel as well. The purpose in assigning a different channel to adjacent cells is to allow distant cells to use the same channel, thereby increasing the effective number of channels.
To facilitate more than a single call within each cell, a cell site base station must have the ability to receive and transmit calls on different channels. At the present time, such a capability is obtained by providing the cell site with multiple transmitter-receiver pairs, each tuned to a particular channel. The disadvantage of this approach is that the cost of providing such equipment rises in direct proportion to the number of channels to be offered. Moreover, each receiver-transmitter pair occupies a certain amount of physical space which is often at a premium within the cell site itself.
Thus, there is a need for a cellular telephone system which can transmit and receive multiple channel using a single transmitter and receiver pair.